1. Field of the Invention
This invention relates to a method of manufacturing a magnetic recording disk, and more particularly to a method of manufacturing a rigid magnetic recording disk formed by applying a flexible magnetic sheet to a surface of a rigid disk base.
2. Description of the Prior Art
There has been known a rigid magnetic recording disk comprising a rigid disk base of aluminum (e.g., JIS A5086) and a magnetic layer formed on a surface of the rigid disk base. The rigid magnetic recording disk is manufactured by machining the rigid disk base, polishing the surface of the machined rigid disk base, and forming a magnetic layer on the surface of the polished rigid disk base by vapor deposition, spincoating or the like.
Generally it is preferred that the surface of the magnetic recording disk is as smooth as possible in order to minimize the space between the disk surface and the magnetic head for recording and reproduction and to effect high density recording and reproduction. However, in the case of the rigid magnetic recording disk having an aluminum disk base, it has been very difficult to obtain a surface having a centerline average height of not larger than 0.1 .mu.m. Further, since the disk base is rigid, it is difficult to continuously apply magnetic layers to disk bases, whereby disk manufacturing cost and manufacturing steps are increased.
Further, though the magnetic head must trace the magnetic layer of the disk while being spaced therefrom by a predetermined small distance during recording or reproduction, it is difficult, in the case of the rigid magnetic recording disk, to keep the small distance constant due to the lack of flexibility of the disk. Further, in the case of the conventional rigid magnetic recording disk, if the magnetic head is brought into contact with the magnetic layer, surface fracture can be produced in the magnetic layer due to the rigidity of the disk base, thereby shortening the service life of the disk. Accordingly, it is very difficult to reduce the distance between the magnetic layer and the magnetic head to further increase the recording density.
Further, the aluminum disk base itself is very expensive.
Thus, there has been proposed a magnetic recording disk comprising a rigid disk base 51 provided with a recess 54 on each side thereof and a floppy disk or a flexible disk 52 bearing a magnetic layer on one side thereof is applied to each side of the rigid disk base 51 over the recess 54 so that a space 53 is formed between the disk base 51 and each flexible disk 52 as shown in FIG. 4.
In the magnetic recording disk of this type, surface fracture of the magnetic layer is less apt to be produced if the magnetic head comes into accidental contact therewith or the magnetic head is brought into contact therewith in order to increase the recording density. Further, since the art of the flexible disk can be applied as it is, and the magnetic layer of the flexible disk which has a high durability and a smooth surface can be used as the magnetic layer, the problems inherent to the conventional rigid magnetic recording disks as described above can be avoided.
In the magnetic recording disk of this type, the manufacturing accuracy of the disk base need not be so high, and accordingly the disk base may be formed by injection molding of plastics or the like in order to improve productivity. When the disk base is formed by injection molding of plastics, the mold for molding the disk base is generally arranged so that molten plastic resin is injected into each cavity through a gate disposed at the center of the cavity to flow radially outwardly from the gate in order to prevent anisotropy being imparted to the molded disk base. The disk base must be provided with a central opening into which the driving shaft of a recording/reproducing system is to be fitted, and the central opening is machined after injection molding of the disk base.
However, this method is disadvantageous in that two steps are required for production of the disk base.
Though the disk base having the central opening can be manufactured only by injection molding step, molten resin must be injected into each cavity through a plurality of gates (which are generally pin gates) in order to avoid anisotropy being imparted to the molded disk base. For example, molten resin must be injected into each cavity through three pin gates 56a, 56b and 56c as shown in FIG. 5. The molten resin injected into the cavity through the three pin gates 56a to 56c fans out to fill the cavity as shown by arrows in FIG. 5. Accordingly, weldlines 57a, 57b and 57c are formed at portions at which the resin flows from the respective gates 56a to 56c encounter each other. The disk base 51 is apt to be strained along the weldlines 57a to 57c and to be lowered in strength along the weldlines 57a to 57c.